1. Field of the Invention
The present invention relates to an Nb-Ti alloy type superconducting wire and a method of manufacturing the same.
2. Description of the Related Art
An Nb-Ti alloy type superconducting wire has a structure comprising a matrix made of Cu or Cu-based alloy in which at least one Nb-Ti based alloy superconducting filament is incorporated.
A method of manufacturing an Nb-Ti alloy type superconducting wire is known which comprises: inserting a member made of an Nb-Ti alloy ingot in a pipe made of Cu or Cu-based alloy; subjecting the pipe to an area-reduction process including extrusion and drawing, thereby to form a composite billet; making a bundle composed of several the composite billets and inserting the bundle in a Cu or a Cu-based alloy pipe; and subjecting the pipe to an area-reduction process including extrusion and drawing, thereby obtaining a final composite. Hereinafter, this method will be referred to simply as "an inserting method".
Recently developed is a method which comprises: embedding a metal such as Nb and Ti which exhibits no superconducting properties in a certain condition--i.e., a temperature of 4.2K, under a magnetic field of several tesla--in advance in a starting Nb-Ti alloy and producing a composite wire in substantially the same manner as described the above. Hereinafter, this method will be referred to simply as "an artificial pinning method".
In the inserting method, a metal phase precipitated from an Nb-Ti alloy matrix called an .alpha.-Ti phase is used as a pinning center to control a principal property of a superconducting wire, that is, a critical current density (hereinafter referred to as "Jc"). On the other hand, in the embedding method, an embedded non-superconducting material is used as a pinning center.
In the inserting method, an appropriate heat treatment accompanied by a cold working is required to allow the .alpha.-Ti phase in the Nb-Ti alloy matrix to be an efficient pinning center. However, since it is difficult accurately to know an amount and a form of .alpha.-Ti phase in a method of manufacturing a superconducting wire, it is very hard to accurately design a superconducting wire having an appropriate Jc value. In addition, this method requires a plurality of complicated and time-consuming processes inevitably raising the manufacturing cost.
On the other hand, in the artificial pinning method, an amount and a change in the form of introduced pinning centers can be estimated. Therefore, it is possible accurately to design a superconducting material having an appropriate Jc value. However, the artificial pinning method has a drawback that a Jc value certainly increases in a low magnetic field of 1 to 3 T, but decreases significantly in a high and middle magnetic field of 4 T or more compared to the inserting method (reported by L. R. Motowidlo et al., Adv. Cryog. Eng., Vol. 36, page 311, 1990; K. Yamafuji et al., Cryogenics, vol. 31 page 431, 1991). A method of preventing the significant decrease of a Jc value in a middle and high magnetic field has not yet been established. This is a main reason why the embedding method has not been widely used in practice compared to the inserting method.